Heme oxygenase catalyzes the first step of heme catabolism, converting a heme molecule to biliverdin. Also of interest is a byproduct of the reaction, carbon monoxide (CO). This toxic gas may serve as a messenger in a manner similar to nitric oxide (NO). A heme oxygenase isozyme, HO-2 is found in neural tissue. The reaction mechanism of heme oxygenase is also of interest, as it may be an example of substrate- assisted catalysis. We have crystals of the heme oxygenase isozyme found in liver and spleen, HO-1, and have collected data up to 2.1 E resolution. Currently no structure is known for any heme oxygenase isozyme.